The present invention relates to an apparatus in which supports in long strip form, such as textiles and plastic sheets (which are hereinafter referred to as "a web" or "webs"), are transported for stretching on a tenter with their trailing edges properly retained. More particularly, the present invention relates to a web transport apparatus in a pin tenter that is used to dry and transport webs with care being taken to prevent them from shrinking in the direction of width.
The web transport apparatus used in conventional tenters is composed of a group of clips or pins that are installed on flat plates connected together to form a chain on either side of a web so that the latter will run along a straight line as it is retained on two lateral sides. This apparatus, which employs a linear transport method, is limited in length and, hence, in the speed at which it can dry and transport webs. However, the drying and transport speed has to be increased in order to reduce the cost of products.
With a view to solving those problems, it has been previously proposed to employ a multi-stage web transport apparatus having reversing sections (I, II) as shown in FIG. 1 (and as disclosed in commonly assigned Japanese Patent Applications Nos. 333110/1988, 650/1989 and 651/1989). Referring to FIG. 1 (although FIG. 1 illustrates the present invention, the same figure will be used for convenience in describing the disclosure of the above applications), a web 1 supported on both sides at point A by means of a pin tenter 2 is transported to a reversing section (I) 3a where it is reversed to change directions, and further travels to the next reversing section (II) 3b where it is reversed again to make another change in direction. Further, the web 1 passes to a separating section 4 where it leaves the pin tenter as it is pulled upward.
The above-described method of drying and transporting webs with an apparatus having multiple reversing sections has suffered from various disadvantages. First, the web is subjected to a very strong tension even if a slight lateral offset occurs on account of shrinkage. Thus, in the reversing sections, particularly in the reversing section (II) 3b where the web rotates with the piercing end of each pin facing inward, the web has a tendency to disengage from the pins so that the degree of the pins' piercing through the web is decreased. Secondly, the shrinking force developed in the web at the reversing sections tends to cause a difference in the transport path between the sides of the web and its central part, whereby the web will deflect toward the center of reversal during transport. This results in web deformation in a pin cushion shape. As a result, external forces act on both sides of the web where they are retained by pins in a different way than when the web is transported only linearly, which can potentially lead to web rupture. Thirdly, when the web is allowed to disengage from the pins at the separating section 4 by pulling it upward as in the earlier case, a strong force is exerted in the areas where the web is retained by pins when it is transported at increasing speeds, which also increases the chance of the web of rupturing at the separating section 4.